Various types of hearing prostheses provide persons with different types of hearing loss with the ability to perceive sound. Hearing loss may be conductive, sensorineural, or some combination of both conductive and sensorineural hearing loss. Conductive hearing loss typically results from a dysfunction in any of the mechanisms that ordinarily conduct sound waves through the outer ear, the eardrum, or the bones of the middle ear. Sensorineural hearing loss typically results from a dysfunction in the inner ear, including the cochlea where sound vibrations are converted into neural signals, or any other part of the ear, auditory nerve, or brain that may process the neural signals.
Persons with some forms of conductive hearing loss may benefit from hearing prostheses, such as acoustic hearing aids or vibration-based hearing aids. An acoustic hearing aid typically includes a small microphone to detect sound, an amplifier to amplify certain portions of the detected sound, and a small speaker to transmit the amplified sounds into the person's ear. Vibration-based hearing aids typically include a small microphone to detect sound, and a vibration mechanism to apply vibrations corresponding to the detected sound to a person's bone, thereby causing vibrations in the person's inner ear, thus bypassing the person's auditory canal and middle ear. Vibration-based hearing aids include bone conduction auditory prostheses, direct acoustic stimulation devices, or other vibration-based devices. A bone conduction auditory prosthesis typically utilizes a surgically-implanted mechanism to transmit sound via direct vibrations of the skull. Similarly, a direct acoustic stimulation device typically utilizes a surgically-implanted mechanism to transmit sound via vibrations corresponding to sound waves to generate fluid motion in a person's inner ear. Other non-surgical vibration-based hearing aids use similar vibration mechanisms to transmit sound via direct vibration of teeth or other cranial or facial bones.
Persons with certain forms of sensorineural hearing loss may benefit from cochlear implants and/or auditory brainstem implants. For example, cochlear implants provide a person having sensorineural hearing loss with the ability to perceive sound by stimulating the person's auditory nerve via an electrode array implanted in the person's cochlea. In traditional cochlear implant systems, an external component of the cochlear implant detects sound waves, which are converted into a series of electrical stimulation signals delivered to the implant recipient's cochlea via the electrode array. Electrically stimulating auditory nerves in a cochlea with a cochlear implant enables persons with sensorineural hearing loss to perceive sound.
A traditional cochlear implant system includes an external speech processor unit worn on the body of a prosthesis recipient and a stimulator unit implanted in the mastoid bone of the recipient. In this traditional configuration, the external speech processor unit detects external sound and converts the detected sound into a coded signal through a suitable speech processing strategy. The coded signal is sent to the implanted stimulator unit via a transcutaneous link. The stimulator unit (i) processes the coded signal, (ii) generates a series of stimulation signals based on the coded signal, and (iii) applies the stimulation signals to the recipient's auditory nerve via electrodes.
In another example cochlear implant, the functionality of the external speech processor unit and the implanted stimulator unit are combined into a single implantable housing to create a totally implantable cochlear implant (TICI). The TICI system can be either a monolithic system containing all the components in a single implant housing or a collection of implant housings coupled together. In operation, detected sound is processed by a speech processor in the TICI system, and stimulation signals are delivered to the recipient via the electrodes without the need for a transcutaneous transmission of signals between an external speech processor unit and an implanted stimulator unit as in the traditional cochlear implant configuration described previously.
Certain types of radio frequency (RF) signals, such as signals generated by magnetic resonance imaging (MRI) systems, present risks for recipients of implantable medical devices such as the cochlear implant devices described above. MRI is a medical imaging technique used to visualize detailed internal structures of a person's body. Because MRI provides good visual contrast between different soft tissues of the body, MRI can be especially useful in imaging the brain, muscles, and heart, for example. In operation, an MRI machine uses a powerful magnetic field to align the magnetization of particular atomic nuclei in the human body, and an RF field to systematically alter the alignment of the magnetization to cause the magnetized nuclei to produce a rotating magnetic field that is detectable by a special scanner. In some circumstances, the RF field generated by the MRI system can induce circulating currents in implant electrode leads at certain RF frequencies. In particular, the electrode leads act as antennas in the presence of the RF field generated by the MRI system. An electrode lead collects RF energy that, in typical systems, is dissipated as heat at localised areas such as the electrode tip. If the electrode tip gets too hot, the electrode tip can damage surrounding tissue and injure the implant recipient. Other types of implantable prostheses having structures or components susceptible to induced currents can pose similar dangers to prosthesis recipients.
Prior approaches for reducing localised heat dissipation to avoid injury to implant recipients have focused on reducing the circulating currents caused by RF fields by changing electrical circuit parameters of the electrode lead in response to MRI-specific frequencies. For example, electrode leads in prior systems have included resonant tank circuits configured as band-stop filters, which resonate and open circuit at a specified frequency. Other prior systems have additionally or alternatively included electromagnetic interference (EMI) filters at the electrode lead ingress and egress to the implant housing to form a low impedance at a specified frequency which in turn causes currents induced by the RF field to be shunted into the implant package.